High Dimensional Thermoelectric Materials—Impact of Alloying on Scattering and Dopant Efficiency

Thermoelectric materials provide a promising platform for transforming waste heat into a sustainable source of electricity. However, the historical paradigm in thermoelectrics has been low dimensional alloys of binary semiconductors, which result in efficiencies inadequate for industrial settings. Recently, high entropy alloys (e.g. Bi-doped (Ge, Ag Sb)(PbTe)) have been found to exhibit improved efficiency¹. Such results are intriguing, as high entropy alloying decreases thermal conductivity while retaining excellent electrical conductivity and mobility. Given the presumed disorder induced in the lattice from high entropy alloying, such excellent transport is surprising.<br/><br/>Our research investigates high-dimensional semiconductor chalcogenide alloys of general form (Ge,Sn,Pb)(S,Se,Te) and Bi-doping therein. Here, we seek to learn how configurational disorder impacts (i) electrical and thermal properties and (ii) extrinsic doping efficiency. To understand the impact of disorder on electronic and thermal properties, we synthesized bulk, polycrystalline samples with composition (Pb_1-x-y))(Te_1-x)(Bi_y)(Ge_0.12Sn_0.32Pb_0.54S_0.12Se_0.27Te_0.61)_xwith 0&lt;x&lt;1 and y = 0.01. For x&lt;0.5, a nearly single phase sample emerges with rock salt structure. The resulting single-phase ingots were characterized to high temperature with Hall, resistivity, Seebeck, and thermal conductivity measurements. Promising figures of merit, zT, were found for heavily alloyed, single phase compounds, largely due to a reduction in lattice thermal conductivity in these highly disordered materials. To complement these experimental measurements, computation was used to investigate the extent of structural distortion within these alloys. DFT calculations were used to train a machine learned force field within NequIP and alloy supercell relaxations were performed with this force field. The extent of structural distortion was quite low, in support of the relatively high mobility of these samples.<br/><br/>These findings motivate us to understand how alloying affects dopant efficiency and, further, impacts the overall figure of merit, zT. While dopant efficiency has been studied in unary and binary semiconductors, it is not clear that these lessons will translate to how dopants behave in high entropy alloys, given the different local environments that the dopant can inhabit. A suite of samples with varying y-values and x = 0.4 were prepared; we measure the carrier concentration to find trends in dopant efficiency and compare these results to Bi-doped x=0 (i.e. PbTe) samples. These Hall measurements were conducted to high temperature to assess if the dopant efficiency varies significantly as the local configurational motifs evolve. Further, we measure a complete suite of thermoelectric properties to assess overall thermoelectric performance at high temperature.<br/><br/>Through these efforts, we have cast light on how increasing structural disorder impacts mobility, lattice thermal conductivity, as well as doping efficiency. These understandings increase our understanding of dopants within high-dimensional spaces, and inform future research in optimizing zT values of thermoelectric materials.<br/><br/>1. Cheng Chang, Mercouri G. Kanatzidis. High-entropy thermoelectric materials emerging[J]. Materials Lab, 2023, 2(1): 220048. doi: 10.54227/mlab.20220048